Fabrication of uniform Ge-nanocrystals embedded in amorphous SiO 2 films using Ge-ion implantation and neutron irradiation methods

Article abstract of DOI:10.1063/1.3553770

Uniform Ge-nanocrystals (Ge-ncs) embedded in amorphous SiO₂ film were formed by using G ⁷⁴e⁺ ion implantation and neutron transmutation doping (NTD) method. Both experimental and theoretical results indicate that the existence of As dopants transmuted from G 74 e by NTD tunes the already stabilized (crystallized) system back to a metastable state and then activates the mass transfer processes during the transition form this metastable state back to the stable (crystallized) state, and hence the nanocrystal size uniformity and higher volume density of Ge-ncs. This method has the potential to open a route in the three-dimensional nanofabrication.

Full text of DOI:10.1063/1.3553770

Fabrication of uniform Ge-nanocrystals embedded in amorphous SiO 2 films using
Ge-ion implantation and neutron irradiation methods
Q. Chen, T. Lu, M. Xu, C. Meng, Y. Hu, K. Sun, and I. Shlimak
Citation: Applied Physics Letters 98, 073103 (2011); doi: 10.1063/1.3553770
View online: http://dx.doi.org/10.1063/1.3553770
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APPLIED PHYSICS LETTERS 98, 073103 ͑2011͒
Fabrication of uniform Ge-nanocrystals embedded in amorphous SiO2 films
using Ge-ion implantation and neutron irradiation methods
Q. Chen,1 T. Lu,1,a͒ M. Xu,2,b͒ C. Meng,3 Y. Hu,1 K. Sun,4 and I. Shlimak5
1Department of Physics and Key Laboratory for Radiation Physics and Technology of Ministry of
Education, Sichuan University, Chengdu 610064, People’s Republic of China
2Institute of Solid State Physics, Sichuan Normal University, Chengdu 610068, People’s Republic of China
and International Center for Material Physics, Chinese Academy of Sciences, Shenyang 110016,
People’s Republic of China
3Key Lab for Shock Wave and Detonation Physics Research, Institute of Fluid Physics,
Chinese Academy of Engineering Physics, Mianyang 621900, People’s Republic of China
4Department of Materials Science and Engineering and Electron Microbeam Analysis Laboratory,
The University of Michigan, Ann Arbor, Michigan 48109-2143, USA
5Department of Physics, Minerva Center and Jack and Pearl Resnick Institute of Advanced Technology,
Bar-Ilan University, Ramat-Gan 52900, Israel
͑Received 9 October 2010; accepted 21 January 2011; published online 14 February 2011͒
74
Uniform Ge-nanocrystals ͑Ge-ncs͒ embedded in amorphous SiO2 film were formed by using Ge+
ion implantation and neutron transmutation doping ͑NTD͒ method. Both experimental and
theoretical results indicate that the existence of As dopants transmuted from 74Ge by NTD tunes the
already stabilized ͑crystallized͒ system back to a metastable state and then activates the mass
transfer processes during the transition form this metastable state back to the stable ͑crystallized͒
state, and hence the nanocrystal size uniformity and higher volume density of Ge-ncs. This method
has the potential to open a route in the three-dimensional nanofabrication. © 2011 American
Institute of Physics. ͓doi:10.1063/1.3553770͔
Research on quantum dots ͑QDs͒ or nanocrystals with a
discrete energy spectrum and size-dependent physical prop-
erties is motivated by potential applications.1–5 Future studies
on smaller technological devices at the nanometer scale will
emphasize techniques for creating nanocrystals with better
size uniformity.6 Photoluminescence ͑PL͒ results show that
the best PL response from Ge-nanocrystals ͑Ge-ncs͒ in amor-
phous silicon oxide films was obtained with samples that
exhibit uniform nanocrystal size.7 Uniform shape and or-
dered size distribution nanocrystals are necessary in
quantum-dot lasers.
In extensively utilized self-assembly methods, the QDs
have a random spatial distribution, and it is difficult to pre-
cisely control dot size.8 Several attempts have been made
recently to improve the uniformity of QD distribution.6,9–14
However, thus far only two-dimensional periodic array uni-
form QDs have been obtained. Achieving uniform QDs un-
der the 3D confinement conditions remains one of the most
daunting challenges ͑in quantum-dot formation, etc͒. Neu-
tron transmutation doping ͑NTD͒ is a technique which uti-
lizes the nuclear reaction of thermal neutrons with the iso-
topes in a semiconductor material,15–17 and it has been
reported that impurities are distributed homogeneously in a
material18–22 by using NTD. In this letter, we report that
Ge-ncs are doped by As atoms by using NTD.
ions accelerated to 150 keV were selected by magnetism
analysis equipment ͑the mass resolution of magnet m/⌬m
=120͒, and implanted into a 640 nm thick amorphous SiO2
film, which was thermally grown on a p-type ͓100͔ Si ma-
74
trix. The implanted dose of Ge+ ions was 1ϫ1017 cm−2.
After implantation, samples were annealed at 800 °C for 0.5
h in a forming gas ͑10% H2 and 90% Ar͒ atmosphere ͑la-
beled as undoped Ge-ncs͒. Neutron irradiation was per-
formed in a nuclear reactor by laying the samples in the
nuclear reactor, with integral thermal neutron fluence of 1
ϫ1020 cm−2. Donor impurity 75As is transmuted from 74Ge
by NTD,
75
74Ge͑n,␥͒ → GeT͑1/2͒_82.8 min
→
75As͑␤−decay͒.
͑1͒
Second annealing is at 800 °C to eliminate any irradiation-
induced defects and to recrystallize the Ge-ncs ͑labeled as
doped Ge-ncs͒.
Cross-section and high-resolution transmission electron
microscope ͑TEM͒ images were obtained with JEOL 2010
FEG transmission electron microscopy. X-ray photoelectron
spectroscopy ͑XPS͒ spectra were measured by Ultra-DLD
XPS, with a mono-Al source operated at a vacuum of 3
ϫ10−9 Torr. The presence of As atoms was proved by the
XPS spectrum. Figure 1͑e͒ shows the low-energy regions of
XPS spectra of the undoped and doped Ge-nc samples. The
broad bands from 23 to 33 eV and 40 to 50 eV correspond to
Ge, SiO2, and As impurities. The positions of the peaks are
25.3 eV for O 2s from SiO2, 29.1 eV for Ge 3d from Ge,
33.2 eV for Ge 3d from GeO2, 40.7 eV for As 3d from As,
44.8 for As 3d from As2O3, and 46.0 eV for As 3d from
As2O5.23 Figure 1͑d͒ shows the size distributions of the un-
doped and doped samples. An area containing approximately
40 dots was analyzed. The mean sizes are 10.2 nm for the
undoped sample and 11.5 nm for the doped sample. The
The preparation of uniform Ge-ncs is as follows. The
isotope nanocrystalline 74Ge samples were prepared by iso-
topic 74Ge-ion implantation, followed by thermal annealing.
Ion implantation was performed in a LC-4 high-energy ion
implanter in a 10−5 Torr vacuum atmosphere. Gas GeH4 was
74
ionized into Ge+ and Ge2+ by arc discharging. Isotope Ge+
a͒
Electronic mail: lutiecheng@scu.edu.cn.
Electronic mail: hsuming_2001@yahoo.com.cn.
b͒
0003-6951/2011/98͑7͒/073103/3/$30.00
98, 073103-1
© 2011 American Institute of Physics
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073103-2
Chen et al.
Appl. Phys. Lett. 98, 073103 ͑2011͒
(c)
(b)
(a)
49 nm
Surface
Surface
65 nm
More uniform
0.3
doped
undoped
Undoped
doped
Ge 3d
0.2
0.1
FIG. 3. ͑Color online͒ Scheme of system considered.
As 3d
GeO2
gions indicate electron accumulation and loss, respectively.
For the O-doping case, since the charges mainly accumulate
in the middle of the bond and are shared with each other,
covalent contact is predominant. For the As doping case,
electron accumulation and loss around the As atom are ob-
vious, as ionic contact is predominant between the As and Ge
atoms in nanocrystal.
0.0
4
6
8
10 12 14 16 18
20
25
30
35
40
45
50
Average Nanocrystals Size (nm)
Binding Energy /eV
FIG. 1. ͑Color online͒ Cross-section TEM images of ͓͑a͒ and ͑c͔͒ undoped
Ge-ncs and ͑b͒ doped Ge-ncs. ͑d͒ Size distributions of the undoped and
As-doped Ge-ncs. ͑e͒ XPS analysis of undoped and As-doped Ge-ncs.
Significant changes were found in the Ge–Ge bond
length after introducing an As atom into the nanocrystal. We
can see that the distortion of nanocrystal is obvious after As
doping. The formation energy of As-doped nanocrystals is
smaller than the O-doped nanocrystals. For example, the av-
average size increases with introducing impurities in the
sample. The size distribution of doped sample ͑7–17 nm͒ is
narrower than the undoped case ͑4–19 nm͒. The insets in
Figs. 1͑a͒ and 1͑b͒ show the cross-section TEM images of
the undoped and doped Ge-ncs. We can clearly see that the
size and distribution of the doped sample are more uniform
than those of the undoped one, and that the volume density
of nanocrystals increased by 13.11%.
To understand the formation mechanism of uniform Ge-
ncs, we have performed interaction studies between As at-
oms and Ge nanocrystal using the ab initio density functional
method performed with the DEMOL3 code,24 in which the
local-density approximation in the scheme of Perdew–Wang
1992 the Perdew-Wang-correlation ͑Ref. 25͒ and the
generalized-gradient approximation in the scheme of
Perdew–Burke–Ernzerhof26 are used. According to the for-
mation energy calculation ͑no shown here͒, an As atom fa-
vors an interstitial site in Ge nanocrystal. Figure 2͑a͒ shows
the ͕220͖ face spatial distribution related to the highest oc-
cupied molecular orbital ͑HOMO͒ of Ge-ncs with an As
atom doping in an interstitial site, where it is the most stable
site of As atom in Ge nanocrystal. For comparison, the spa-
tial distribution of the O case is shown in Fig. 2͑b͒. The
HOMO is especially important in determining the chemical
reactivity of a system. We can see that the HOMO states
progressively localize on the impurities. Red and blue re-
¯
erage Ge–Ge bond length ͑L͒ increases to 2.68 Å, 4.69%
higher than the undoped case ͑2.56 Å͒. However, the average
value for O-doping is 2.55 Å, indicating that doping with As
produces a greater distortion. The deflection of impurity at-
oms from the center of local symmetry of doped nanocrystal
interstitial sites ͑labeled as ⍀ center͒ was also found. The
distance increases from 2.89 Å ͑between the ⍀ center and
the Ge atom in the center of nanocrystal͒ to 3.13 Å ͑between
the As atom in interstitial site and the Ge atom in the center
site of nanocrystal͒ after the As doping and geometry opti-
mization. Otherwise, for the O-doping in the same interstitial
site, the distance between the O atom and the center of the
nanocrystal is 2.04 Å after geometry optimization. A large
repulsion force between the As atoms and Ge-ncs is indi-
cated.
A possible formation mechanism of the Ge nanolayer
may be discussed as follows ͑see Fig. 3͒. Lu et al.27 insisted
that the Debye length is given by
2
ͱ
lD = ␧skBT/͑q Nd͒,
͑2͒
where ␧ is the permittivity of the substrate, kB the Boltz-
s
mann constant, T the temperature, q the charge of one
electron, and Nd the donor density in the substrate. When two
Ge-ncs approach each other, the accompanying charge
clouds overlap, leading to a repulsive force that prevents
them from coalescing. Significant repulsion appears when
the separation of two Ge-ncs reduces to a scale comparable
to lD. An energy barrier appears at a separation of approxi-
mately 0.02lD, below which van der Waals interaction
dominates and drives two Ge-ncs into contact. A silicon sub-
As
(a)
(b)
O
strate has ␧s=3.9␧0, where ␧ is the permittivity of vacuum.
0
Using ␧0=8.85ϫ10−12 F/m, q=1.60ϫ10−19 C, kB=1.38
ϫ10−23 J/K, T=300 K, and Nd=5ϫ1013 cm−3, we have
lD=334.1 nm. In Fig. 1͑a͒, we cannot see the separation of
FIG. 2. ͑Color online͒ Spatial distribution related to the HOMO of ͑a͒ As-
and ͑b͒ O-doped Ge-ncs.
Ge-ncs from the inset TEM image, and the distance between
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073103-3
Chen et al.
Appl. Phys. Lett. 98, 073103 ͑2011͒
the two Ge-ncs is about 10 nm, much smaller than the cal-
culated Debye length. This leads us to infer that there may be
some other mechanism of uniform nucleation.
tem back to a metastable state and then activating the mass
transfer processes during the transition from this metastable
state back to the stable ͑crystallized͒ state, nanocrystal size
uniformity was improved. We report a method that creates
semiconductor Ge quantum dots with high uniformity and
high density, showing promising applications in three-
dimensional nanofabrication.
Followed by the 74Ge ion implantation, primary thermal
annealing, neutron irradiation, and a subsequent reannealing,
the nucleation mechanism of As-doped Ge-ncs is extraordi-
narily complex, including damage-enhanced diffusion and
interfacial-energy effects. Nonetheless, the size distributions
become more uniform after introducing As into Ge-ncs. The
local high-temperature nucleates Ge ions into Ge-ncs after
Ge-ion implantation. Annealing due to the minimum energy
principle is then carried out, because the Gibbs free energy
of a nanoparticle aggregated from ions is much lower than
the energy sum of these ions. The dopant atoms are usually
not in their lattice positions but displaced in interstitial posi-
tions or isolated points due to the recoil produced by the ␥-
and ␤-particles and the lower formation energy in Ge-nc
interstitial sites. After irradiation, As separates from Ge-ncs
and creates more nucleation sites. Due to the large repulsion
force between As atoms and Ge-ncs, the As nucleation sites
are 3D positional uniform. In addition to producing As do-
nors, the neutron irradiation shakes and breaks the Ge-ncs,
which becomes amorphous state, introducing more defects.
The irradiated samples were subjected to the second anneal-
ing to eliminate any irradiation-induced defects and to re-
crystallize the Ge-ncs around the As nucleation center. There
are counterbalances between nucleation and spinodal decom-
position during nanocrystal formation. During the process of
nucleation, each Ge nanocrystal in samples deforms laterally
to match the amorphous SiO2 substrate. The deformation in-
duces a tensile stress region in the substrate below. Accord-
ing to the calculation, the distortion of Ge-ncs is larger after
As doping, as shown in Fig. 2͑a͒. Doping with As leads to
greater charge cloud density and distortion. The strain fields
from misfit dislocation make the sizes and spacing of sample
more uniform.
This project was supported by the NASF of NSFC-
CAEP of China ͑Grant No. 10376020͒, the Program for New
Century Excellent Talents in University ͑Grant No. NCET-
04-0874͒, and Sichuan Youth Science & Technology Foun-
dation, China ͑Grant No. 08ZQ026-025͒. We thank Dr.
Levchenko and Professor Ostrikov at CSIRO Material Sci-
ence and Engineering of Australia for their helpful discus-
sion.
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